THURSDAY, July 20, 2017 (HealthDay News) -- In the long-standing debate over whether antidepressants are safe to take during pregnancy, a new study suggests that exposure to the drugs in the womb might bump up a child's risk of autism.

The risk of autism was 45 percent higher for kids whose moms took antidepressants compared to kids born to mothers with psychiatric disorders who weren't prescribed antidepressants, the study found.

"We found consistent results pointing towards a small effect of antidepressants with autism, especially higher functioning forms of autism without intellectual disability," said lead researcher Dheeraj Rai. He is a senior lecturer in psychiatry with the University of Bristol in the United Kingdom.

"We think it is important to keep in mind the absolute risk, which is small," Rai said. "Over 95 percent of women in the study who took antidepressants during pregnancy did not have a child with autism."

Also, the study wasn't designed to prove a cause-and-effect relationship, and only found an association between antidepressants and the risk of autism.

"It turns out you would prevent 2 percent of autism cases" if no pregnant women ever took antidepressants, Frazier said.

"It's definitely not a major contributor," Frazier added. "It appears to be a significant and real relationship, but it's not a strong one in terms of how many cases of autism this would prevent."

Another recent study, published online July 12 in JAMA Psychiatry, had better news for depressed moms-to-be. That research found no statistically significant difference in intellectual disability in children born to mothers taking antidepressants compared to kids whose moms didn't take the drugs.

For the current study, Rai and his research team analyzed data from more than 254,000 children aged 4 to 17 living in Stockholm between 2001 and 2011.

Almost 5,400 of the children were diagnosed with autism. More than 3,300 of the children were exposed to antidepressants during pregnancy. And more than 12,000 children were born to mothers with psychiatric disorders who weren't taking an antidepressant in pregnancy, the study revealed.

About 4 percent of children exposed to antidepressants had been diagnosed with autism. Meanwhile 2.9 percent of children born to a woman with a history of psychiatric problems who didn't take antidepressants during pregnancy developed autism.

Antidepressants were more strongly associated with cases of autism that didn't include intellectual disability, the researchers said.

No one is certain why antidepressants might be linked to autism, but most antidepressants act by increasing brain levels of a neurotransmitter called serotonin, Rai and Frazier said.

"Serotonin is important in brain development and higher levels have been observed in children with autism," Rai said. He added that animal research has shown that exposing animal fetuses to antidepressants that boost serotonin levels causes autism-like symptoms in the animals.

At the same time, Rai said it is too early to rule out the possibility that depression itself increases the risk of autism, rather than the medications prescribed to treat depression.

"It is known that chronic forms of stress could affect brain development of the fetus," Rai said. "There is also the possibility of a common genetic pathway -- for example, women who have a higher genetic risk of autism could also be more likely to have severe depression and be prescribed antidepressants."

Rai and Frazier also agreed that pregnant women with depression should not stop taking their medicine without first talking it over with their doctor.

"They should not base decisions about the use of antidepressants during pregnancy on any one study, especially when the research evidence is conflicting, as in this case where different studies have reached different conclusions," Rai said.

"There could be severe risks of stopping or not taking antidepressants during pregnancy both to the mother and the fetus, so the benefits of these medications for mothers who need them should not be forgotten," he said.

Rai concluded that this study should be seen as a "small effort contributing to the understanding of the complex mechanisms behind autism," rather than evidence that antidepressants can in and of themselves cause autism.